Various systems have been proposed in which a railroad truck comprises a pair of wheelsets with each of the wheelsets being attached to the truck in a manner that allows the wheelsets to move to a radial configuration when the truck is travelling on curved railroad tracks. One such example is Canadian Pat. No. 1,083,886 issued Aug. 19, 1980, to Urban Transportation Development Corporation Limited. Other examples include U.S. Pat. No. 2,071,207 to Heinrich Knecht (1935) and U.S. Pat. No. 356,347 to Brown and Midelton issued 1887.
In the foregoing patents and indeed in most systems providing a two axle truck having steerable axles the axles are mounted to the truck so as to be pivotal about a substantially vertical pivotal axis. When the truck enters a curve, the axles pivot about their respective pivotal axes so as to be radially aligned. Radial alignment is desirable for many reasons including reduction of wear of components and noise which would otherwise be caused by slippage of the wheels along the rail. It is convenient to refer to a truck having steerable axles as a steerable truck.
Although several mechanisms have been provided to permit radial alignment of axles very little thought has been given to preventing divergent behaviour of these axles when the truck is travelling along the track. By reason of minor variations in the rails or other problems there may be slight disturbances which would tend to turn the truck from its desired position. Analysis must be carried out to determine whether the truck will return to its desired position, once deviated from it, or whether it will continue to deviate further from the desired position until there is contact between the flange of the usual railroad wheel and the rail.
In most mechanisms including that illustrated in the aforesaid Canadian patent, the truck is attached to the railroad vehicle body for relative pivotal movement about a substantially vertical pivotal axis. In the ordinary case, where it is desired that the axles share the load equally, and the pivotal connection between the truck and the car body also carries the load of the car to the truck, the pivotal connection is located centrally between the axles. In some situations where the designers did not intend the wheels to share equal loads (such as when only one axle is driven) the pivotal axis is offset from the point midway between the axles.
We have found that steerable trucks are not necessarily stable. That is, when such steerable trucks are disturbed from the desired position they may not return to the desired position but rather continue to deviate until there is contact between the rail and flange of the wheel. We have found that it is possible to stabilize such a steerable truck by offsetting the pivotal axis between the truck and the railroad vehicle body by a certain critical amount. Alternatively, stability may be achieved in such a truck by providing resilient spring-like forces to restrain pivotal movement of the truck with respect to the car body alone or in combination with offset of the pivotal axis. We have discovered that there is an important mathematical relationship between the conicity of the wheels of the standard railroad vehicle forming part of the typical wheelset, the amount of offset of the pivotal axis from the midway point between the two axles of such a truck, and the resilience of the pivotal connection between the truck and the car body of the vehicle.